A Synergy Of Different Corrosion Failure Modes Pertaining To Structural Steels In Liquid Lead-Bismuth Eutectic Flow

Lead bismuth eutectic(LBE)is one of the candidate coolants for Generation ?fast reactors.Due to its favorable physical and chemical properties,LBE earns itself greater consideration for improving the safety of nuclear reactor.However,the compatibility issue of LBE with structural steels raised great concern for its industrial application.It is generally accepted that two fundamental liquid metal corrosion(LMC)mechanisms,i.e.dissolution and oxidation,proceed in static liquid LBE,depending on the dissolved oxygen concentration.Although corrosion tests of structural steels in flowing LBE have been performed worldwide,most dynamic corrosion tests are still limited to laminar LBE flow.Meanwhile,there are great challenges in construction and maintenance of dynamic corrosion setup.Thus,erosion behaviors under extreme LBE flow patterns,as well as the potential interaction mechanism between erosion and traditional corrosion failure modes,were rarely reported so far.In current work,a modified dynamic corrosion setup was built up.And test conditions were no longer confined to an impact angle of 0°.T91 ferritic/martensitic steel and 304N austenitic stainless steel were selected for dynamic corrosion tests at 400? up to 1000 hours.For T91 ferritic/martensitic steel,the precipitation/dissolution equilibrium of external oxide scales was described in terms of element diffusion or mass transfer kinetics,the latter of which were simulated by computational fluid dynamics(CFD).A special viewpoint of high surface curvature-enhanced oxide growth stress was put forward to account for co-existence of a porous magnetite layer and a dense one within erosion cavities,which can be treated as concave surface.From the above viewpoint,we also proposed a new cracking mechanism to account for the tendency of preferential cracking at the inflection of concave/convex surface.Such cracks across the oxide scales constituted the first access for LBE penetration.EBSD analysis indicated that gradient deformation bands within steel subsurface significantly affected LBE penetration paths.By means of high-resolution transmission electron microscopy(HRTEM),nanoscale Cottrell atmosphere accompanied by high density of edge dislocations close to the scale/alloy interface was revealed.The role of Cottrell atmosphere in oxidation of alloys was discussed for the first time.For perpendicularly eroded 304N austenitic stainless steel,erosion-corrosion behaviors were characterized by X-ray diffraction(XRD),Scanning Electron Microscopy(SEM),TEM,Raman spectroscopy and Electron Back-scattered Diffraction(EBSD)methods.We advanced a special perspective of subsurface degradation in chemical compositions and mechanical properties.Chemical degradation leads to local breakaway oxidation,while mechanical degradation leads to dissolution along local crack paths.Even in LBE flowing at a fairly low rate of 1 m/s,alteration in impact angle from 0° to 90° was proven to result in an abrupt increase in erosion rate for both steels,as well as a combined effect of different corrosion failure modes.A better understanding of their synergy provides guidance for material design and corrosion protection techniques for primary pump impeller suffering from extreme LBE flow pattern in lead fast reactor.